[PATCH] Fix agent code generate bug of ref

[Hui Zhu <teawater at gmail dot com>]

Hi,

I meet a issue with tracepoint and Linux kernel is:
(gdb) list update_curr
590		cfs_rq->load_unacc_exec_time += delta_exec;
591	#endif
592	}
593	
594	static void update_curr(struct cfs_rq *cfs_rq)
595	{
596		struct sched_entity *curr = cfs_rq->curr;
597		u64 now = rq_of(cfs_rq)->clock;
598		unsigned long delta_exec;
599	
(gdb)
600		if (unlikely(!curr))
601			return;
602	
603		/*
604		 * Get the amount of time the current task was running
605		 * since the last time we changed load (this cannot
606		 * overflow on 32 bits):
607		 */
608		delta_exec = (unsigned long)(now - curr->exec_start);
609		if (!delta_exec)
(gdb) trace 609
Tracepoint 1 at 0xffffffff8104ced6: file
/home/teawater/kernel/taobao-kernel/tmp/linux-2.6.32-220.23.1.el5/kernel/sched_fair.c,
line 609.
(gdb) actions
Enter actions for tracepoint 1, one per line.
End with a line saying just "end".
>collect now
>end
(gdb) tstart

Then it will failed in kernel part, the address that send to kernel to
collect is 0x978.  But the right address of now is 0xffff880002215578.

I check the agent code that is got is:
(gdb) maintenance agent -at sched_fair.c:609, now
Scope: 0xffffffff8104ced6
Reg mask: 20
  0  reg 5
  3  const16 128
  6  add
  7  ref8
  8  const16 2232
 11  add
 12  const8 8
 14  trace
 15  end

I think ref8 is not right because this acode try to get a address from
Linux kernel.  It should be ref64.
I check the code of function dwarf2_compile_expr_to_ax:
	case DW_OP_deref:
	case DW_OP_deref_size:
	  {
	    int size;

	    if (op == DW_OP_deref_size)
	      size = *op_ptr++;
	    else
	      size = addr_size;

	    switch (size)
	      {
	      case 8:
		ax_simple (expr, aop_ref8);
aop_ref8 means ref 8 bits.  So use addr_size is not right, I add first
patch fix-op_deref-size.txt to change addr_size to addr_size_bits.
Then the first issue is fixed.

And I found that GDB generate right code the collect value from
0xffff880002215578.

But after that, I still got error when I tfind:
(gdb) tfind
Found trace frame 0, tracepoint 1
#0  update_curr (cfs_rq=0xffff880002214d28, cfs_rq@entry=<error
reading variable: PC not available>)
    at /home/teawater/kernel/taobao-kernel/tmp/linux-2.6.32-220.23.1.el5/kernel/sched_fair.c:609
609		if (!delta_exec)
(gdb) p now
Cannot access memory at address 0xffff880002214da8

This issue is because aop just collect value of now, but not ref
address.  But GDB need this value.
So I add second patch trace_def_if_trace.txt to call ax_trace_quick if need.

Then the aop will be changed to:
(gdb) maintenance agent -at sched_fair.c:609, now
Scope: 0xffffffff8104ced6
Reg mask: 20
  0  reg 5
  3  const16 128
  6  add
  7  trace_quick 8
  9  ref64
 10  const16 2232
 13  add
 14  const8 8
 16  trace
 17  end

Then all the issue of this tracepoint is fixed.  But I failed with
make a test code with it.  So I just can repeoduce in this Linux
kernel code.

I suggest the next release just this 2 patches because this 2 issue
will affect some code of aop.

Thanks,
Hui



2013-03-10  Hui Zhu  <hui_zhu@mentor.com>

	* dwarf2loc.c (dwarf2_compile_expr_to_ax): Change addr_size to
	addr_size_bits if DW_OP_deref.


2013-03-10  Hui Zhu  <hui_zhu@mentor.com>

	* dwarf2loc.c (dwarf2_compile_expr_to_ax): Call ax_trace_quick
	if need.

Attachment: fix-op_deref-size.txt
Description: Text document

Attachment: trace_def_if_trace.txt
Description: Text document